1. Field of the Invention
The present invention relates to a method for manufacturing an optical module, where the optical module provides an optical device with a semiconductor laser diode and a lens for concentrating the light output from the laser diode. In particular, the method relates to optically coupling the optical device with a stub including a coupling fiber whose end surface facing the lens is inclined with respect to the optical axis of the coupling fiber.
2. Related Prior Arts
For the optical module that combines the optical device including the semiconductor optical device and the lens with the optical receptacle that includes a sleeve and a stub, an optical alignment between the optical device and the optical receptacle is inevitable, because a misaligned module with an inferior optical coupling therebetween degrades the signal-to-noise ratio to shorten the transmission range.
The U.S. Pat. No. 5,963,696 has disclosed a method for optically aligning the optical receptacle with the optical device. That is, the method disclosed therein first calculates an optimum incident angle α for the inclined angle θ of the tip surface of the coupling fiber. Next, assuming the gap between the lens and the LD to be a, the displacement x0 is calculated as x0=a·tan(α). Subsequently, the LD chip is mounted on the package so as to target a position displaced by x0 from the center S of the package, while the lens is mounted on the package so as to align the center of the lens with respect to the center of the package. Finally, the process fixes the optical fiber as aligning the direction of the inclined end surface thereof with the direction of the displacement and obtaining a position where the maximum coupling efficiency is realized by sliding the optical receptacle within a plane perpendicular to the optical axis.
The LD module, which is often called as a transmitter optical sub-assembly (TOSA), generally integrates a stub with an inclined end surface by 6° to 8° to the optical axis to eliminate the light returning the LD. On the other hand, it is necessary for optically coupling the light output from the optical device with a good coupling efficiency to incline the optical axis of the light with respect to the axis of the stub. This peculiar configuration between the optical receptacle and the optical device may be realized, for example, by assembling the LD chip as inclining the axis thereof, or by offsetting the lens in horizontally, namely, perpendicular to the optical axis, with respect to the LD.
Thus, it is necessary for the optical alignment between the optical receptacle with the stub and the optical device to align the inclined direction of the end surface of the stub with the direction of the light beam from the optical device, which is inevitable to realize the optimum optical coupling therebetween and the superior performance for the wiggle characteristic, where the wiggle is the fluctuation of the optical coupling efficiency depending of the suspended state of the optical fiber.
Conventionally, the direction of the light beam output from the optical device was determined primarily based on the designed displacement between the LD and the center of the lens. However, the manufacturing process inevitably involves a substantial tolerance in physical dimensions of components and the positional accuracy thereof. Thus, the direction of the light beam output from the optical device strongly depends on the relative position of the lens to the LD. Paradoxically, to control the relative position of the lens to the LD may adjust the direction and the angle of the output beam from the optical device.
However, it is practically hard to control the relative position of the lens to the LD, that is, to suppress the positional deviation of the lens from the designed point within a desired range. The current assembling process using the image recognition technique for the LD chip realizes the positional deviation of the chip within a desired range. The aligning process for the lens with respect to the lens holder also realizes the deviation within an acceptable range. However, the welding of the lens holder to the stem of the package involves the positional deviation of about thirty and forty micron meters. In addition to this positional deviation of the lens holder, the misalignment and the inclination of the LD chip, and the inclination of the sub-mount may also generate the discrepancy between the practical direction and the designed direction of the optical output from the optical device. Accordingly, the direction of the output beam from the optical device scatters from 90° to 120°.
Thus, the present invention is to provide a method for manufacturing an optical module even when the components therein involves the positional deviation and the optical output appears an inconsistent direction with the designed parameter.